The known types of data protection methods use a cryptographic algorithm comprising execution cycles of repetitive operations for processing data elements contained in a memory of the card so as to generate encrypted information intended to be communicated to the computer terminal.
The execution of the method by the microprocessor of the card results in the sending of derivative signals such as peaks in the level of the microprocessor's electric power consumption, or variations in the electromagnetic radiation such that the envelope of electromagnetic radiation is indicative of the data processed. An attacker seeking to use the microprocessor cards in an unauthorized way can trigger the execution of the method repeatedly and analyze the derivative signals emitted in order to determine correspondences between the various processing operations and each signal or series of signals. From these correspondences, and for example by subjecting the card to electromagnetic disturbances or voltage drops at precise moments in the execution of the algorithm, the attacker can study the encrypted information obtained and the differences, or lack of differences, between the derivative signals emitted, in order to discover the data contained in the memory of the card.
To complicate this type of analysis of the derivative signals, it has been suggested that parasitic signals be generated and added to the derivative signals emitted during the execution of the method. The extraction of the signals that correspond to the execution of the method is then more difficult, but it is still possible. It has also been suggested that the electronic components of the card and the program for executing the method be designed so that the derivative signals emitted are independent of the value of the sensitive data. However, this complicates the production of the cards without providing satisfactory protection of the data.